The present invention relates to gaskets for use in sealing, lightning strike protection and EMI shielding applications, and particularly for aircraft related applications. More particularly, the present invention relates to a resilient crimped electrically-conductive mesh sheet encapsulated with a polymeric elastomer gel polymer for use in shielding and protecting external aircraft components which generate or receive electromagnetic radiation, as well as protecting internal computer systems, avionics and instrumentation.
In general, electronic components are sources of electromagnetic (EM) radiation. Electronic components, for example, transmitters, transceivers, microcontrollers, microprocessors and the like radiate a portion of the electric signals propagating through a device as EM radiation. The EM radiation generated in this way is sometimes referred to as EM noise. Higher operating frequency ranges of the electronic components leads to the EM noise that primarily comprise radio frequency (RF) radiation. This RF radiation is normally referred to as RF noise. As may be used herein, EM noise and RF noise are used merely to refer to EM radiations emitted from an electronic device. Moreover, EM noise and RF noise, unless otherwise stated, may be used interchangeably throughout the specification. EM radiation may also be emitted from electronic devices in close proximity to each other (EMI, or electromagnetic interference).
Electronic devices have been conventionally shielded to impede the emission of EM noise. Specifically, the electronic devices can be enclosed in a shield. The shield may be made of various materials, for example, metal sheets, plastic composites, conductive polymer sprays, metal filled epoxy pastes and the like. The shield absorbs EM radiation thereby impeding the emission of EM noise from an assembly of the electronic devices and the shield.
Composite gaskets generally comprising a metal core material enclosed or encapsulated within a resilient polymeric material are well known in the art. Such gaskets have sufficient structural integrity to be useful in sealing components in corrosive and high performance environments, such as for pressure vessels, automotive engines and aircraft. Examples of such gaskets include U.S. Pat. Nos. 3,230,290; 4,865,905; 5,791,654; 5,929,138; 6,357,764; 6,454,267; 6,530,577; 6,695,320; and 6,719,293. Composite gaskets may also include EMI shielding capabilities, such as those disclosed in U.S. Pat. Nos. 2,477,267; 3,126,440; and 4,900,877. The disclosure of each of these patents is incorporated by reference herein in their entirety.
Many of the gaskets described in the aforementioned patents may not be acceptable for high performance applications, typically aircraft applications, where a variety of performance characteristics may be required in harsh working environments. For example, in addition to EMI shielding and sealing, electrical bonding of components and protection against corrosion may be a necessity.
Conventional gaskets typically have electrical contact only at the edge portion of the gasket. Most such gaskets involve a woven flat wire mesh buried within the body of the elastomer not near the surface. When the gasket is cut to size, the wire mesh is exposed at the edge of the gasket and bent up near the surface. Since the electrical contact of these gaskets and the sealed components is at the edge portion of the gasket, a caulk must be applied at the edge of the gasket to protect the wire mesh from corrosion while maintaining the electrical bonding and EMI shielding. The application and curing of the caulk requires several hours of application and curing time, increasing down time of the aircraft, for instance. When a gasket is replaced, the old caulk must first be removed, and the removal procedure can result in scratches to the protective coating of the aircraft, requiring a repainting of the aircraft surface, thereby expanding the scope and duration of the repair. In addition, most caulking compounds have a limited shelf life which can create inventory obsolescence and increase associated costs.
In addition to the aforementioned applications, conductive gaskets can also be used for aircraft protection against lightning strikes. Estimates show that a typical commercial airliner averages one lightning hit per year. Generally, the first contact with lightning is through the nose, wingtip or antenna, with the lightning exiting through an extremity such as the tail. Lightning strikes can adversely impact the onboard avionics, such as computers and flight instruments. The problem can be exacerbated on modern aircraft which typically employ composite materials in place of aluminum for cost and weight reduction. These composites tend to be poor electrical conductors and are susceptible to damage in the event of a lightning strike.
One way to dissipate the effect of lightning strikes is to dissipate the charge build up through the use of static dissipaters. An aircraft antenna can function as a static dissipater if property insulated from the aircraft frame. The present sealing and EMI shielding gasket can also function as an effective lightning strike dissipation element when used in combination with external avionics components such as aircraft antenna mounts. The conductive elements of the gasket can function to dissipate the static charge build up on aircraft lightning strike zones.
Accordingly, it be advantageous to have a commercial product with a greater deflection range, requiring the application of less closure force, fewer attachment bolts, and thinner mating flanges. Thinner flanges and fewer bolts lead to reduced weight which is important for aircraft applications. Additionally, a gasket with a greater deflection range would be able to provide environmental sealing between mating surfaces which may have a substantial lack of conformity for protection against environmental leakage.
It will be appreciated that there is a need for an improved composite gasket design, particularly in aircraft applications, that would provide for improved sealing, lightning strike protection and EMI shielding, while necessitating fewer overall repair and maintenance problems.